1. Field of the Invention
The present invention concerns a laminated glass for a transportation vehicle, in particular, a laminated glass for an airplane, and specifically, a laminated windshield for an airplane.
2. Discussion of the Background
In aircraft, laminated glass generally comprises at least two sheets of thermally or chemically tempered glass having a thermoplastic polymer insert layer. The insert layer used typically consists of several plies or layers of plasticized polyvinyl butyral (hereinafter designated as PVB). PVB has a high modulus of tension and a considerable breaking stretch (percent elongation at break). Furthermore, PVB acts as an energy absorber, and can assure the tightness and integrity of a laminated glass in which all the glass sheets are broken.
Under the usual conditions of a high-altitude flight, such a laminated glass is subjected to substantial differences in pressure and in temperature between its two surfaces (i.e., the surface facing inside the aircraft and the surface facing the external atmosphere). PVB has a thermal expansion coefficient well above that of the glass sheets. Consequently, the piece of glass is subjected to substantial shearing stresses, principally at the edges of the glass. These high stresses may be further increased when the glass is struck, particularly at low temperature (for example, when the windshield of the airplane in flight strikes a bird). At temperatures of less than 0.degree. C., PVB offers a greatly increased modulus of tension to aid in withstanding such impacts.
The problem associated with stresses is still more pronounced when the insert contains, countersunk in its edges, a peripheral band of metal or a stratified material. The metal or stratified material increases the rigidity of the glass, and thus permitting or facilitating, as the case may be, installation of the glass in the opening in the body of the cockpit, by bolting for example.
Solutions have been proposed to reduce the stresses cited above, and consequently to lessen the risks of glass failure. One of the proffered solutions consists of applying a material acting as a separator in the marginal area of the glass. This solution may be suitable for pieces of glass of small dimensions. It generally is not satisfactory, however, for pieces of glass the size of an airplane windshield. Furthermore, the separator materials may promote the penetration of moisture in the PVB insert, leading to losses in adhesion in undesirable places, as well as losses in the optical qualities, particularly the transparency, of the glass. In addition, the separator materials suggested to date do not have the optical quality of the typical materials used in laminated glass. Thus, use of such separator materials reduces the field of vision of the glass.
It also has been suggested in French Patent Publication FR-A-2,310,979 to place a continuous film of polyurethane between the PVB layer and the sheet of glass. The polyurethane film deflects or diminishes the shear forces on the glass sheets and polyurethane interlayers, thus reducing the adverse effects of thermally-induced contraction and stresses on the PVB.
This solution also has drawbacks. It generally requires an adhesion-activating process for the polyurethane over the entirety of its contact surface with the glass. Furthermore, the supplementary interface between the two different thermoplastic products (PVB and polyurethane) may lead to optical defects in the glass, because each of these polymeric materials have different indexes of refraction. Even further, thermal stresses may lead to a cleavage of chemically tempered sheets of glass from the thermoplastic polymer inserts, in the plane parallel to the contact surface with the glass.